Voters across the United States are casting ballots in a pivotal election that will determine whether Democrats or Republicans control Congress during President Barack Obama's final two years in office.

All 435 seats in the House of Representatives and a third of the Senate's 100 seats are at stake. The main focus is the Senate, where many political analysts say Republicans are poised to win the six seats necessary to seize control of the chamber from Obama's Democratic Party, which holds a narrow 55-seat majority.

With the president's approval rating mired in the low 40 percent range, the Republican Party's best chances are in several states that Obama lost two years ago during his re-election campaign.

Republicans are expected to retain -- and perhaps even expand -- their solid 233-seat majority in the House.

The latest opinion surveys show Republican candidates will easily win Senate races in West Virginia, South Dakota and Montana, which are currently held by Democrats. Polling also shows Republicans poised to win Senate races in Iowa, Colorado and Alaska, although it may be several hours -- or even days -- before the final results are known.

Two races in the South, Georgia and Louisiana, are so close it is expected they will require runoff elections.

Senate Republican Leader Mitch McConnell, himself embroiled in a competitive re-election race in Kentucky, expressed confidence about his party's chances during a final campaign appearance Monday in Louisville.

"Victory is in the air and we're going to bring it home tomorrow night!" he promised supporters.

In a widespread campaign tactic, Republican Senate candidates have sought to link Democratic incumbents to Obama's unpopularity. Numerous Democratic officeholders declined to invite the president to campaign for them and often cited instances where they disagreed with him, such as on health care or energy issues.

The accuracy of pre-election U.S. political surveys has often been erratic, with some polling turning out to be way off the mark. Even as several Senate races are deemed too close to call, analysts say Republicans have about a 70 percent chance of picking up at least six seats to control the Senate.

If Republicans do control Congress, it could presage new disputes with Obama over his signature legislative achievement, massive national health care reforms that have allowed millions of people to secure insurance coverage they could not previously afford. Many Republicans view it as excessive government involvement in peoples' health care and call for repeal of the law.

Many Republicans also attacked Obama's handling of the current Ebola crisis, called for approval of an oil pipeline from Canada through the central U.S. and a curb on government regulation of businesses. Some opposition lawmakers have also disputed the president's handling of Russia's intervention in Ukraine and U.S. airstrikes against Islamic State militants in Iraq and Syria.

In the United States, the two main political parties are feuding over spending and tax policies and immigration reforms.

Obama has vowed to set new immigration rules by executive order by the end of the year, after the House did not act on comprehensive reforms approved by the Senate. Some Republicans already are saying they will seek to block the president from unilaterally changing the country's immigration policies to allow millions of migrants who entered illegally to stay in the United States.